The present invention relates to a power control circuit for use with a transmitter, in particular, to a power control circuit having a power amplifier for linearly amplifying a signal and outputting the amplified signal.
In, for example, an AM modulating method performed by a transmitting system, the control of power fluctuation of a transmission signal may be an important factor for improving the performance. A transmitter used in such a transmission system has an RF (Radio Frequency) power amplifier that linearly amplifies a signal and outputs the amplified signal.
In addition, the RF power amplifier should amplify a signal in a linear region so as to prevent the signal from distorting.
However, due to various causes such as a temperature change, the gain of a pre-stage amplifier of the RF power amplifier may fluctuate, thereby causing the input level of the RF power amplifier to fluctuate. If the RF power amplifier operates in the saturation region due to the fluctuation, it cannot linearly amplify the input signal. Thus, since the signal largely distorts, required spurious standard cannot be satisfied.
To solve this problem, a saturation preventing control loop method is known. In this control method, an output signal of the RF power amplifier is detected. The detected output voltage is compared with a reference voltage. The pre-stage variable gain amplifier is controlled corresponding to the relation between the output voltage and the reference voltage. Thus, the output power of the RF power amplifier is controlled so that the RF power amplifier does not operate in the saturation region. FIG. 1 is a schematic diagram showing the construction of a circuit according to this control method.
In the power control circuit that operates according to the saturation preventing control loop method, a modulated signal that includes speech information and the like, that are input from an input terminal 1, is amplified by a variable gain amplifier 2. The amplified signal is supplied to an RF power amplifier 3. The RF power amplifier 3 amplifies the input signal with a predetermined amplitude. The amplified modulated signal is supplied from the RF power amplifier 3 to a signal output terminal 5 through a directional coupler 4. The directional coupler 4 extracts a part of the modulated signal. The extracted modulated signal is detected by a detecting circuit 6. A comparator 8 compares the level of the detected signal with a predetermined reference voltage supplied from a reference voltage supply terminal 7. The comparison result of the comparator 8 is fed back to the gain of the variable gain amplifier 2. Thus, the level of the modulated signal that is supplied from the variable gain amplifier 2 is adjusted.
In other words, when the level of the detected signal is higher than the reference voltage, the gain of the variable gain amplifier 2 is decreased so as to decrease the level of the modulated signal that is supplied to the RF power amplifier 3. When the level of the detected signal is lower than the reference voltage, the gain of the variable gain amplifier 2 is increased so as to increase the level of the modulated signal that is supplied to the RF power amplifier 3.
The saturation preventing control loop 20 always properly maintains the level of the modulated signal that is supplied to the RF power amplifier 3, thereby preventing the RF power amplifier 3 from operating in the saturation region.
However, the circuit shown in FIG. 1 only controls the output power of the saturation preventing control loop method according to the reference voltage supplied from the reference voltage supply terminal 7.
For example, as in a CDMA (Code Division Multiple Access) cellular system that should control transmission power for maintaining the maximum system capacity, the above-described variable gain amplifier should have a control signal corresponding to the required transmission power.
However, in the circuit shown in FIG. 1, the transmission power can be controlled according to the reference voltage supplied to the reference voltage supply terminal 7. In this method, however, due to the response time and output power detecting characteristics of the loop, the system standard cannot be adequately satisfied.
The present invention is made to solve such a problem. An object of the present invention is to prevent an RF power amplifier used to perform a conventional saturation preventing control loop method from saturating in the case that the system transmits a signal with maximum power, and to disconnect the saturation preventing loop for controlling the transmission power in the case that the system transmits a signal without the maximum power so as to satisfy characteristics required for the power control method of the system.